The present disclosure relates to apparatuses and methods for reducing the buildup of bottom dross in a zinc bath and reducing the transition time between two bath states.
Galvanizing (GI) and galvannealing (GA) are two known processes. Galvanization is a chemical process that is used to coat steel or iron with zinc in order to reduce corrosion (specifically, rusting). In galvannealing, steel or iron that has been coated with zinc is then heated (annealed) to improve fabrication and corrosion resistance characteristics.
Continuous galvanizing or galvannealing is typically done by running a steel or iron sheet through a molten zinc bath contained in a coating pot. The zinc bath contains zinc (Zn), aluminum (Al), and iron (Fe) and usually has a temperature of 450-480° C. (840-890° F.). Zinc is the overwhelming component of the zinc bath. The aluminum content of the zinc bath ranges from 0.10 weight percent (wt %) to 0.4 weight percent. In GI, the aluminum content of the zinc bath is greater than 0.13 wt %. In GA, the aluminum content of the zinc bath is less than 0.13 wt %. In another related process called galvalume, the zinc bath contains 55 wt % Al and 45 wt % Zn. The iron content is usually very low (less than 0.1 wt %) and generally comes from the steel sheet itself.
The zinc-rich field of the Zn—Fe—Al phase diagram is helpful for understanding the chemical processes that occurs during GI and GA. In particular, the phase field changes around 0.13 wt % Al at these temperatures and different impurities (i.e. intermetallic compounds) occur in different phase fields. GA is usually operated within the δ+L phase field, wherein the impurity is FeZn7(δ). This impurity is denser than the zinc bath itself and collects on the bottom of the coating pot; thus, it is also known as bottom dross. GI operates within the η+L phase field, wherein the impurity is Fe2Al5(η). This impurity is less dense than the zinc bath itself and collects on the surface of the molten zinc bath in the coating pot; thus, it is also known as top dross. These impurities generally form because the solubility limit of Fe is reached in a local region. Dross particles can thus nucleate and grow.
The bottom dross and top dross are undesired. Whereas the top dross can be continuously removed by skimming the top of the zinc bath, the bottom dross cannot. Continued operation of the GA process thus builds up bottom dross, which can solidify. In addition, the bottom dross (FeZn7) consumes the desired zinc reactant in the zinc bath. This aspect is also undesired.
Bottom dross can be removed. If the bottom dross has solidified, it can be mechanically removed by jack-hammering; however, this usually results in a week of downtime. Bottom dross can also be removed using scoops before it solidifies, but this method is dangerous, tedious and still results in downtime.
Bottom dross can be removed chemically by exploiting the differences between GA and GI. Aluminum is added to the zinc bath as solid ingots to change the phase field from δ+L to η+L. This allows the bottom dross (FeZn7) to convert to top dross (Fe2Al5), which can then be skimmed off. However, this method of removing bottom dross has its own disadvantages. Typically, the transition time during which the bottom dross converts to top dross is 24-30 hours. During this transition time, the continuous galvanizing line produces only products having significantly lower value.
Galvannealed steel is widely used in the automobile, appliance, and construction industries because of its comparatively superior corrosion resistance properties. Thus, it would be desirable to continually run a galvannealing process or, at a minimum, reduce the transition time between the GA to GI processes.